This invention relates to composite materials for use in blades of fluid flow machines and the like and, more particularly, to increasing the impact tolerance of such materials.
For many years attempts have been made to replace the relatively heavy, homogeneous metal blades and vanes of fluid flow machines such as gas turbine engine compressors with lighter composite materials. The primary effort in this direction has been toward the use of high strength, elongated filaments composited in a lightweight matrix. Early work involved glass fibers, and more recent efforts have been directed toward the utilization of boron, graphite and other synthetic filaments. These later materials have extremely high strength characteristics as well as high moduli of elasticity which contributes to the necessary stiffness of the compressor blades and vanes.
Many problems have confronted the efforts to utilize these filaments, particularly in adapting their unidirectional strength characteristics to a multidirectional stress field. To a large extent, these problems have been overcome and composite blades have been demonstrated with performance characteristics, in many areas, equal to or better than their homogeneous metal counterparts in addition to providing the expected and significant weight reductions.
However, one major obstacle to the realization of the full potential of composite materials for gas turbine engine applications has been their relatively low tolerance to impact damage or foreign object damage (FOD) due to foreign object ingestion. Typically, a composite blade is fabricated by bonding together a plurality of substantially parallel filament laminates. Each laminate consists of a single layer of generally elongated filaments anchored in a lightweight matrix. Where, for example, the matrix comprises aluminum and the filaments are boron, aluminum foil sheets are placed on both sides of the boron filament layer and bonded together by the known diffusion bonding or continuous-roll bonding technique.
Under certain processing conditions in composite blade manufacturing, the degree of bonding can be extensive, resulting in a rigid structure incapable of tolerating high impact loadings. Since the matrix material cannot absorb much energy through deformation, and since the laminates are extensively bonded, substantially all of the load is carried by the filaments which are relatively hard and brittle. Fracture of the filaments generally results in fracture of the blade. Higher impact strength matrix materials, on the other hand, do not possess the bondability of the more ductile materials. If bonding is substantially incomplete between filament laminates, the laminates tend to slide with respect to each other under shear loadings, much in the manner of a deck of cards. When such sliding occurs, ability to absorb impact energy greatly decreases. Increases in bonding pressure and temperature, though effective in increasing bonding, can produce crushing of the filaments and high residual thermal stresses due to the different coefficients of expansion of the various constituents. Both of these factors contribute to reduced impact resistance and, thus, reduced tolerance to foreign object impact damage.
Thus, it becomes desirable to develop a composite material which is adaptable to the environment of a gas turbine engine rotor blade by exhibiting improved tolerance to foreign object impact. Such a material should exhibit the tolerance to impact of known higher-impact strength materials and yet possess the bondability of high fatigue strength materials.